1. Field of the Invention
The present invention relates generally to optical communications, and more specifically to fiber-based devices and methods for providing low-loss spectrally periodic filtering of an optical signal.
2. Technical Background
Soliton optical communication systems can greatly benefit from the use of narrowband spectral filters spaced at appropriate intervals along the optical fiber within the system. Narrowband filters pass through the useful spectral content of an optical signal and reject broadband ASE (amplified-spontaneous-emission) noise. By “guiding” the central frequency of the soliton pulses to the middle of the filter passband, such filters can significantly reduce the accumulation of jitter in soliton pulse arrival times caused by Gordon-Haus and acoustic-interaction effects. For this reason, such filters are often known as frequency-guiding filters. See, for example, P. V. Mamyshev, “Solitons in optical fiber communication systems”, in Fiber Optics Handbook, McGraw-Hill, 2003.
It has also been suggested that narrowband spectral filters, inserted at certain intervals between segments of nonlinear positive dispersion fiber, can significantly increase the dynamic range of a nonlinear-fiber-based dispersion compensator. See, for example, U.S. Patent Publication Number 2004-0184815, published 23 Sep. 2004, and assigned to the assignee of the present case. Similar to soliton transmission, the increase is achieved by limiting the signal distortion and timing jitter that arises from the nonlinear mixing of the optical data signal and broadband ASE noise.
For application in wavelength-division-multiplexed optical transmission systems, frequency-guiding or noise-rejecting spectral filters need to have multiple passbands, one for each channel. Since WDM channels are typically positioned at standard frequencies corresponding to the ITU grid, with spacing of 50 GHz or an integer multiple thereof (typically 100, 200, or 400 GHz), the same relatively small periodicity is required of a bandpass filter.
To reduce the amount of additional signal amplification required in a soliton-transmission application, it is desirable to use frequency-guiding filters with low insertion loss. For use in dynamic dispersion compensation applications, low filter loss is critical, since optical signal power must be kept close to the fundamental soliton power at every point within the nonlinear fiber.
Several technologies have been used or suggested for producing low loss periodic (frequency or spectrum-periodic) filters. One well-known technology is based on fiber Mach-Zehnder interferometers, using the so-called BFT (biconic fused tapered) couplers. In a typical BFT Mach-Zehnder interferometer, two dissimilar fibers are fused and tapered together at two different points. Approximately half of the light entering the first fuse point in one of the fibers is transferred to the second fiber. The interferometer is completed at the second fuse point, at which the light is coupled back into the first fiber. Unfortunately it is difficult to manufacture this type of device with bandpass periodicity as small as 50-400 GHz, because this requires a relatively large fiber length between tapers.
Periodic filters can also be made using planar waveguide interferometers, but small spacing between transmission peaks is difficult to achieve with this technology as well.
Filters with small spectral periods can be made using thin-film Fabry-Perot etalons or bulk optics based Mach-Zehnder interferometers, but they typically have an insertion loss on the order of 1 dB or even higher because of the additional optics required to collimate the light beam from the input fiber and to couple it into the output fiber.
Modal interference within multi-mode fiber has also been used to produce spectrally-periodic (or “comb”) filters. For example, in A. J. Poustie, N. Finlayson, P. Harper, “Multiwavelength fiber laser using a spatial mode beating filter”, Optics letters, v. 19, No. 10, pp. 716-717, 1994, a comb filter is formed within a laser cavity by splicing a section of multi-mode fiber between two sections of a single-mode fiber. Similarly, in Q. Li, C-H Lin, P-Y Tseng, H. P. Lee, “Demonstration of High Extinction Ratio Modal Interference in a Multi-mode Fiber and Its Applications for All-Fiber Comb Filter and High-Temperature Sensor”, Optics Communications 250 (2005), 280-285, modal interference within a multi-mode fiber was used to produce a spectrally periodic filter with relatively small period. But insertion loss (>2 dB) and filtering performance were less than optimal for soliton-based systems, and particularly for dynamic dispersion compensation.
A need thus exists to develop suitable filters and suitable methods for producing filters with relatively high bandpass periodicity (of 400-50 GHz, or even tighter spacing) together with low insertion loss and sufficient extinction ratio (of at least 10 dB or more).